Detonating Shard

Make a melee spell attack roll, on a hit you implant a shard of ice in the body of an ennemy.  You can than detonate the shard using a bonus action on subsequent turns.  On the 1st turn it deals 2d8 pts of damage, on the 2nd turn it increases to deal 3d8 pts of damage and the 3rd round it increases by 1d8 every round afterward.

Higher Spell Slots: Add 1d8 per spell slot added to the 3rd round of damage.